WP 8.1 not supported anymore.

https://blogs.unity3d.com/2017/07/11/introducing-unity-2017/
https://docs.unity3d.com/Manual/windowsstore-sdks.html

To reproduce:
1. Import attached picture. It is 2048x1024 (so POT)
2. In the texture settings set it to advanced. Set the sprite mode to single. Make sure generate mip maps is on
3. On the texture platform settings for wp8, select "Override for WSA"
4. Set the max size to 2048
5. Set the format to ARGB 32 bit
6. Add the sprite to scene
7. Build & Run for WP8.1
8. Turn the device sideways to switch to landscape - observe that the texture is now some sort of salad

Alternatively, build & run the attached repro2.unitypackage and turn the phone sideways.

Seems to happen on weaker GPUs like on Nokia Lumia 520